The present application pertains to a low temperature sputter target/backing plate bonding method and to the assemblies made thereby.
Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires gas ion bombardment of a target having a face formed of a material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a heat conducting backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials traverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
In addition to the use of an electrical field, increasing sputtering rates have been achieved by the concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent the target surface thereby increasing the number of electron-gas atom collisions in the area to produce an increase in the number of positively charged gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded (i.e. consumed for subsequent deposition on the substrate) in a generally annular section of the target face, known as the target raceway.
In conventional target cathode assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in well-known disposition in order to form the above-noted magnetic field in the form of a loop or tunnel extending around the exposed face of the target.
In order to achieve good thermal and electrical contact between the target and backing plate, these members are commonly attached to each other by means of soldering, brazing, diffusion bonding, clamping and by epoxy cement and the like.
In some prior art target/backing plate assemblies, projections formed in either the interfacial surfaces of the target or backing plate are received in corresponding recesses or grooves of the other member to improve thermal conductivity. For example in Hillman U.S. Pat. No. 4,885,075, annularly arranged members of high thermal conductivity protrude from either the target or backing plate into a corresponding recess formed in the other member. Upon heating of the target during sputtering, the protruding members expand radially to make contact with the recess wall to thereby improve heat transfer between the target and backing plate.
Elastic and plastic deformation of backing plate members in target/backing plate cathode assemblies is minimized in accordance with U.S. Pat. No. 5,269,899 (Fan), of common assignment herewith, by the provision of mating teeth like projections disposed along the target/backing plate interfacial surfaces, with the mating surface of the backing plate having a concave surface. Here again, upon heating of the target, the target teeth expand radially and make snug contact with the mating teeth on the backing member to enhance thermal conductivity.
U.S. Pat. No. 5,230,459 (Mueller et al.) of common assignment herewith teaches the use of diffusion bonding methods wherein one of the target/backing plate mating surfaces is first prepared with grooves or the like. During the diffusion bonding process, jagged portions of the grooves penetrate into the metal from the opposed interfacial surface and disrupt the formation of oxide and other bond inhibiting layers that may otherwise form along the target/backing plate interface to thereby improve bonding efficacy. The method results in a strong bond with increased shear resistance such that the assembly can withstand thermal stress that otherwise may tend to result in debonding or target warpage during use.
In those cases in which diffusion bonding of Al, Cu, or other low melting point targets have been tried, the heat needed for the diffusion bonding process causes an undesirable growth in grain size. Accordingly, there is a need in the art to provide a low temperature target bonding method so that target grain size can be accurately controlled and maintained at a desired minimal size.
In order to meet the above need, a target and backing plate are provided wherein the harder metal of the two is machined or otherwise formed so as to have a plurality of ridges or other salient surface portions thereof. These ridges or salient portions are formed along the interior surface space of the target or backing plate. The ridged surface is then placed alongside the mating surface of the other member of the assembly along a target/backing plate interface wherein bonding will occur.
The peripheral portion of the interfacial surface of the assembly is bonded by conventional means such as E-beam welding, TIG welding, friction welding, etc., preferably under vacuum. The assembly is then pressed together at a low temperature, such as room temperature, so that the target and backing plate are in contact to improve thermal conductivity of the assembly.
The projections or ridges formed along either the target or backing plate penetrate into the mating metal surface along the target/backing plate interface. These projections penetrate the softer metal on the opposing mating surface breaking through the oxide surface thereon. This results in a metal to metal cold diffusion type bond with the projections bending as a result of the cold pressing and forming a mechanical interlocking mechanism.
After the cold pressing operation, a low temperature annealing can be performed on the assembly to enhance adherence of the mating surfaces.